In 1923, Fischer and Tropsch, German chemists, developed a Fischer-Tropsch synthesis method (F-T synthesis method), and this method has enabled the production of liquid hydrocarbon from coal, natural gas, biomass and the like by way of syngas. A process producing liquid hydrocarbon from coal is referred to as a coal-to-liquid (CTL) process, a process producing from natural gas is referred to as a gas-to-liquid (GTL) process, and a process producing from biomass is referred to as a biomass-to-liquid (BTL) process, and recently, similar processes are collectively referred to as XTL (“X” resource-to-liquid) processes.
These processes first convert each resource material (coal, natural gas, biomass and the like) into syngas using methods such as gasification and reforming, and the composition of the syngas suited for XTL processes for producing liquid fuel preferably has a hydrogen to carbon monoxide ratio of approximately 2 as shown in the following Reaction Formula 1.CO+2H2+—[CH2]—n→—[CH2]—n+1+H2O  [Reaction Formula 1]
The ratio of hydrogen exceeding 2 is not preferable since it increases the selectivity on methane, and consequently, the selectivity on C5+ (hydrocarbon having 5 or more carbon atoms) relatively decreases. In Reaction Formula 1, olefins and oxygenates (molecules including oxygen atoms such as alcohols, aldehydes, carboxylic acids and ketones) are also produced as byproducts in addition to hydrocarbon having linear chains as shown above.
One of the main purposes of XTL processes is to obtain liquid fuels, and therefore, the latest trend is to decrease selectivity on methane and to produce linear hydrocarbon, in particular, C5+ linear hydrocarbon with high selectivity by optimizing a selection of reaction catalyst, a syngas ratio, a temperature, a pressure and the like. Herein, cobalt-series catalysts are normally used as the reaction catalyst, and such metal catalysts are used by being uniformly dispersed and supported on the surface of a support such as alumina, silica and titania. For the improvement of catalyst performances, noble metals such as Ru, Pt and Re may be used as a co-catalyst.
Such catalysts are normally used by being supported by a support such as alumina (—Al2O3, a-Al2O3 and the like), silica (SiO2), titania (TiO2) and magnesia (MgO). However, the use of silica materials having mesoporous structures such as SBA-15 and MCM-41, and carbon-based materials having mesoporous structures such as CMK-3 and carbon nanotubes has also been expanded recently. An incipient wetness method, an impregnation method and the like are normally used for such supports. For example, a target mass ratio of the catalyst material is supported in the pores of the support while repeatedly performing processes of dissolving a cobalt salt of acid (Co(NO3)2.H2O and the like), i.e., a catalyst precursor, and a salt such as Pt, Ru and Re used as a co-catalyst in proper solvents to prepare a mixed solution of the precursor, and impregnating the mixed solution of the precursor in the pores of the support, followed by drying. After that, the dried catalyst goes through a calcination process under air or inert gas atmosphere, and catalyst particles having a form in which cobalt oxide crystals are supported in the support are obtained.
A Fischer-Tropsch catalyst shows activity in a reduced metal state, and therefore, a syngas needs to be supplied in a pure metal state after sufficient reduction processes before reaction.
In a laboratory-scale experiment for developing catalysts, an in situ reduction method, in which the temperature is raised up to a reduction temperature while flowing a reducing gas with a calcinated catalyst to be filled into a reactor, is normally used. However, commercial reactors often employ other methods since reduction temperatures are generally much higher than reaction temperatures, and separate reducing gas injection equipment is required for an in situ reduction method.
In commercial processes, reduction is carried out by supplying reducing gas (a mixture of hydrogen and an inert gas where the hydrogen content is approximately 5 to 10%) with an additional catalyst reduction equipment. Cobalt metals in a reduced state violently react with oxygen in air and are oxidized again. Therefore, a proper treatment is necessary to not expose cobalt metals to air, or to minimize the degree of oxidation when exposed. Such a treatment is referred to as passivation, and by an intentional mild oxidation of the surface only through the supply of a mixed gas (normally consisting of oxygen and an inert gas) with a low concentration of oxygen, the activity of a catalyst can be minimally degraded when exposed to air during its transfer.
However, the passivation method has several problems. First, the degree of proper passivation is very difficult to identify. The degree of oxidation treatment required for minimizing violent oxidation during the air exposure is different for each catalyst. In addition, there are problems that initial activity is not satisfactory since oxidation has been partially progressed before use, and activity is generally low compared to an in situ reduction method.
In order to solve such problems, S. Hammache et al. (S. Hammache, J. G. Goodwin, Jr., R. Oukaci, Catalysis Today, 2002, 71, 361-367) designed a passivation method using CO gas or (CO+H2) gas. However, the method has a problem in that the activity of a catalyst is degraded due to the production of graphitic carbon on the surface of the catalyst, and additionally, the method further requires a heating equipment capable of being operated at high temperatures in a reactor since the reduction process includes treating the catalyst with hydrogen gas for 10 hours at a high temperature of 350° C. when activating a carbide compound catalyst.
In addition, F. Huber et al. (F. Huber, H. Venvik, Catalysis Letters, 2006, 3-4, 211-220) proposed an encapsulation method using organic materials, a carbon layer coating method, a method of passivating metal catalysts through oxygen and N2O treatments. However, for activating, the method also requires reduction conditions of heating for 16 hours at a high temperature of 350° C. while supplying hydrogen gas.
Furthermore, various passivation methods carried out through the production of carbide and carbon have also been proposed in prior documents. WO 03/002252 discloses a method for transferring or activating a catalyst by passivating the activated catalyst using a method of coating the surface of a metal precursor material supported in a support with carbon by adding a certain amount (5 to 20%) of short-chained hydrocarbon (methane, ethane, etc.) together with hydrogen gas, or introducing a syngas, in order to produce a carbide form of a metal catalyst in a hydrogen reduction process.
Metal catalysts having a carbide form are known to have increased activity after activation, and the activity is known to be further improved when a metal carbide form is formed in certain parts of an activated metal catalyst. However, WO 03/002252 discloses that hydrogen reduction treatment at a high temperature of 350° C. or greater is necessary to activate the catalyst passivated in a metal carbide form, thus requiring an additional activation equipment in addition to the reaction equipment.
Meanwhile, even when an ex situ reduction method is used, a method without passivation by oxygen, that is, a method of introducing a catalyst directly into a reactor without being exposed to oxygen at all may be considered. However, the method also has problems. The catalyst reduction equipment and the reactor need to be relatively close, and the equipment may become larger since gas supply equipment, power, a heater and the like required for reduction all need to be included in a reactor system. In addition, there is a new challenging task on how to transfer solid particles from a catalyst reduction equipment to a reactor.
In order to solve these problems, Sasol Limited and Shell Oil Company and the like have devised methods in which, by coating a reduced catalyst with wax or inserting the reduced catalyst inside a wax material, the transfer of the catalyst becomes simple while capable of blocking its contact with air. However, because wax materials are solid at room temperature they need to be liquidified by heating in order to coat or insert catalyst particles, thus making the process complicated.
Meanwhile, an activated metal catalyst produces water and hydrocarbon compounds having various chain lengths by being reacted with a syngas. Performances of metal catalysts in a Fischer-Tropsch synthesis reaction are determined by the distribution of metal catalyst active sites and the degree of activation (degree of reduction), the interaction between catalysts and supports, the content of impurities in a syngas, and the degree of inactivation due to reaction products (wax and water) in the reaction, and performances of Fischer-Tropsch catalysts are determined by catalyst preparation methods, the choice of support/metal material and the presence of co-catalysts.
In the transformation of carbon monoxide, the performance of a cobalt catalyst is one of the most important factors in evaluating the economic feasibility of XTL processes. Particularly, hydrogenation reaction conversion ratio (CO conversion ratio) of carbon monoxide, which is a carbon source supplied by a syngas, and selectivity (C5+ selectivity) of a linear hydrocarbon product having 5 or more carbon chains capable of being used as liquid fuels are important factors in evaluating catalyst performances. However, in addition to these, the degree of catalyst deactivation due to long-time operations greatly affects the choice of industrial catalysts in real processes. The main reasons for the deactivation of a cobalt metal catalysts in low temperature Fischer-Tropsch reactions carried out at 200 to 250° C. and 15 to 25 bar include the degradation of catalyst active sites due to catalyst poison such as sulfur and nitrogen compounds included in a syngas, the oxidation of cobalt metals, the formation of compounds between cobalt metals and supports, the degradation of active sites due to the sintering of small cobalt metal crystals, the recrystallization of atomic structures in cobalt metal surface, and the carbon deposition on active sites of cobalt, etc. Among these reasons, the main reasons for catalyst deactivation in real commercial plants include, along with the oxidation of the catalyst, the degradation of catalyst active sites due to long-chained wax products, and the blockage of catalyst pores, the degradation of active sites due to carbon deposition, the degradation of active sites due to the sintering of cobalt particles, and friction due to the collision of catalysts.
In real processes, the causes of catalyst deactivation such as sintering of cobalt particles, carbon deposition, and catalyst friction may be suppressed by changing reaction conditions of Fischer-Tropsch synthesis. For example, the sintering of cobalt particles occurs due to a sudden temperature increase in a catalyst unit in a circumstance where heat is not controlled, and cobalt particles supported in a support at a low Fischer-Tropsch reaction temperature condition of 210 to 230° C. almost do not experience sintering, therefore, catalyst deactivation due to sintering may be prevented when the heat generated from an exothermic reaction is effectively removed. Additionally, the degradation of catalyst activation due to carbon deposition may be diminished when a syngas with a H2/CO ratio of 2 is used as a reactant at proper reaction temperatures and pressures, since catalyst deactivation due to carbon deposition occurs when the ratio of carbon monoxide is high in the syngas, or the reaction is carried out at high temperatures and high pressures. In addition, activity degradation due to catalyst attrition may be diminished when a fixed-bed reactor or a slurry bubble column reactor having no catalyst damages due to impellers is used.
However, the degradation of active sites due to the wax produced by a reaction, and the degradation of catalyst activation due to the blockage of catalyst pores occur due to products. Therefore, the catalyst activity is gradually degraded reversibly according to the duration of reactions unless the reaction is stopped.
Accordingly, in the catalyst of which activity is degraded due to a long-time Fischer-Tropsch synthesis reaction, wax remaining on the catalyst surface blocks the active sites of the catalyst and is impregnated inside the pores. Therefore, reactivation is required to remove the wax and regenerate the active sites.
As existing reactivation methods of removing wax on a catalyst surface, US Patent Application Publication No. 2004/0259963 discloses a method for regenerating a catalyst including separating a catalyst present on slurry from the slurry for reactivation, de-waxing for removing wax, oxidizing the catalyst to an oxide form from a cobalt metal state, and reducing the catalyst again, thereby reactivating catalyst. In addition, US Patent Application Publication No. 2010/0081562 discloses a method for reactivating a catalyst including injecting a wax-coated catalyst into a pumpable liquid suspension, reactivating the catalyst by putting into contact with a high-temperature gas flow of 600 to 1050° C. for approximately 0.01 to 10 seconds, and separating the reactivated catalyst from the gas flow. US Patent Application Publication No. 2005/0124706 discloses a method including heat-treating a cobalt catalyst deactivated due to a long-term use several times for 4 hours at 300° C. under the atmosphere formed with 7 volume % of hydrogen and 93 volume % of vapor, and reactivating the catalyst to have 95% activity with respect to the initial catalyst activity through a carbon monoxide hydrogenation reaction.
However, the existing catalyst reactivation methods are inconvenient in that a catalyst needs to be reinjected after going through a process of extracting the catalyst inside a reactor out of the reactor and washing the catalyst when a fixed-bed reactor is used. When the reactivation is carried out inside a reactor to avoid such inconvenience, the temperature conditions of a reactivation reaction carried out by oxide formation and hydrogen are higher than Fischer-Tropsch synthesis reaction conditions, and consequently, the sintering of a cobalt metal catalyst may be induced. In addition, there are disadvantages in that the reactivation process including extraction and washing, oxidation, and reduction is rather complicated, and additional equipment needs to be installed around the reaction equipment.
Meanwhile, when a catalyst is regenerated in a fluidized-bed reactor, external equipment for separating the catalyst and fluidized liquid needs to be installed. Even when reactivation is carried out inside a reactor, the loss of fluidized liquid may occur when a reactivation temperature is above the boiling point of the fluidized liquid (350° C. or higher). In addition, there is a disadvantage in that, for the catalyst regeneration, costs for equipment for catalyst separation, regenerated gas supply, and heating additionally occur.
Existing catalyst regeneration methods have been developed with steps of catalyst separation, catalyst washing, catalyst oxidation, and catalyst reduction. Methods for simultaneously carrying out oxidation and reduction for simplifying the regeneration method, or for regenerating a catalyst inside a reactor without catalyst separation and washing have been proposed. However, only the number of steps in the process for catalyst regeneration decreases, and all the proposed methods commonly require a high-temperature heating of 300° C. or higher for final catalyst regeneration.